Chemistry note for CTEVT diploma level : Organic Chemistry (IV)

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Chapter- 6 : Alkyl halides 

Laboratory preparation of chloroform (trichloromethane) (CHCl3) :

Principle:  Chloroform is prepared in the laboratory by heating ethanol or acetone with aqueous bleaching powder paste. Bleaching powder paste acts as oxidizing, chlorinating and hydrolyzing agent.

 

From ethanol :

Step I : Oxidation :

Step II : Chlorination :

 

Step III : Hydrolysis :

 

From acetone (propanone) :

Step I : Chlorination :

 

Step II : Hydrolysis :

 

lab preparation of chloroform

Procedure : First of all, bleaching powder paste is prepared by mixing 100 gm of bleaching powder with 200 ml of water in one liter round bottomed flask and 25 ml of ethanol or acetone is added to it. The flask is heated gently on a water bath until a mixture of chloroform and water distills over, as shown in figure . The mixture from receiver is transferred into a separating funnel and the lower layer of chloroform is separated.

Purification : The impure chloroform is washed with dilute caustic soda (NaOH) solution and then with water successively in the separating funnel. It is then dried over anhydrous calcium chloride and redistilled between 60 – 650C. In this way, pure and dry chloroform is obtained.

Chemical properties of chloroform :

 1. Reaction with air : In the presence of sunlight, chloroform is oxidized by air to produce highly poisonous gaseous compound called phosgene (carbonyl chloride).

 

Thus, chloroform is stored in a dark/coloured bottle to prevent the oxidation of chloroform into phosgene.

 2. Reaction with aq. KOH solution : When boiling with aqueous KOH solution, chloroform is hydrolysed to form potassium formate which on acidification gives formic acid.

 

 3. Reaction with silver powder : Chloroform when heated with silver powder gives acetylene(ethyne).

 

 4. Reaction with primary amines ( carbylamine reaction) : When chloroform is warmed with a primary amine in the presence of alcoholic KOH, an offensive(unpleasant) smell of carbylamines ( i.e. isocyanide) is obtained. This reaction is known as carbylamine reaction.

 

 5. Reaction with phenol (Riemer- Tiemann reaction) : When chloroform is heated with phenol and sodium hydroxide followed by hydrolysis, o – hydroxy benzaldehyde ( salicylaldehyde) is formed. This reaction is called Riemer – Tiemann reaction.

 

 6. Reaction with acetone(propanone) : Chloroform reacts with acetone in presence of a base such as KOH to give chloretone.

 

Chloretone is used as a sleep – inducing (hypnotic) drug.

 7. Reaction with HNO3 : On heating with conc. HNO3, chloroform gives chloropicrin.

 

Chloropicrin is used as an insecticide and tear gas.

Uses of chloroform :
  1. It is used as an anesthetic. It is now being replaced by other safe anesthetics because chloroform in some cases causes cardiac and respiratory failure.
  2. It is used as a laboratory reagent for testing primary amines.
  3. It is used for the preparation of chloropicrin, chloretone, salicylaldehyde, etc.
  4. It is used in medicines such as a cough syrups.
  5. It is used as preservative for biological specimens.

Chapter- 7 : Alcohol

Classification of alcohols :

 1. Monohydric alcohol : Alcohols which contain only one – OH group are called monohydric alcohols.

   Eg. CH3 – CH2 – OH (ethanol)

 2. Polyhydric alcohol : Alcohols which contain more than one – OH groups are called polyhydric alcohols. Eg.

  • Dihydric alcohol : Alcohol which contains two – OH groups is called dihydric alcohol. Eg.

       

  • Trihydric alcohol : Alcohol which contains three – OH groups is called trihydric alcohol. Eg.

       

Monohydric alcohols are further classified as primary, secondary and tertiary alcohols according to the nature of – OH bonded carbon atom.

 i. Primary alcohol (10 alcohol) : Alcohols in which – OH bonded carbon atom is further bonded with one or none other carbon atom are called primary alcohols. Eg.

  CH3 – CH2 – OH (ethanol)

  CH3 – OH (methanol)

 ii. Secondary alcohol (20 alcohol) : Alcohols in which – OH bonded carbon atom is further bonded with two other carbon atoms are called secondary alcohols.

 

 3. Tertiary alcohol (30 alcohol) : Alcohols in which – OH bonded carbon atom is further bonded with three other carbon atom are called primary alcohols.

 

Preparation of alcohols :

 1. By hydrolysis of haloalkanes : Alcohols are produced when haloalkanes( alkyl halides ) are treated with aqueous sodium or potassium hydroxide.

 

 2. Industrial preparation of ethanol by fermentation of carbohydrates (molasses) :

Fermentation is a biochemical process of degradarion ( slow decomposition/ breaking down) of large organic molecules like sugars and starches into simpler compounds by the catalytic action of enzymes.

Molasses is the dark coloured liquid left after crystallization of sugar from sugar cane juice. The reactions occurring during the fermentation of sugar(molasses) are :

 

Chemical properties of ethanol 

 1. Action with sodium metal (acidic nature) :

Ethanol reacts with sodium metal to form sodium ethoxide with the evolution of hydrogen gas.

 

 2. Action with phosphorus halides (halogenation):

Ethanol reacts with phosphorus halide to form haloethane. Eg.

 3. Action with conc. H2SO4 :

When ethanol is heated with conc. H2SO4, it undergoes dehydration to give either ether or ethene.

 

 4. Action with carboxylic acid (esterification reaction) :

Alcohol reacts with carboxylic acid in presence of conc. H2SO4 to give ester.

 

 5. Oxidation of alcohols :

Identification of primary, secondary and tertiary alcohol by oxidation method :

Alcohols are oxidized by different oxidizing agents like acidic or alkaline KMnO4, acidified K2Cr2O7, dil. HNO3, etc. to give different products.

 i) Primary alcohols are easily oxidized first to aldehyde and then to carboxylic acids containing same number of C- atoms as in parent alcohol. Eg.

 

 ii) Secondary alcohols on oxidation give ketones with same number of carbon atoms. The ketones are further oxidized only under drastic conditions ( i.e. prolong treatment of oxidizing agent) to give carboxylic acid containing lesser number of carbon atoms. eg. 

 

 iii) Tertiary alcohols do not contain hydrogen atom on the carbon carrying – OH group (i.e. α- hydrogen). Thus in order to oxidize tertiary alcohol, a carbon-carbon bond must be broken. For this reason 30 alcohol do not undergo oxidation reaction in neutral or alkaline medium.

 

But if the oxidation is carried out in the acidic medium under drastic condition tertiary alcohol oxidize to give a mixture of ketone and carboxylic acid. The ketone thus formed further gets oxidized to carboxylic acid. eg.

 

⇐ See previous chapters

       See next chapters ⇒

References :